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POWER GENERATION AND TRANSMISSION                  59
24-hr, service. The customer pays for an estimated power requirement, which must not be exceeded, and pays whether he uses the power or not. Meter rates are usually based on consumption: the greater the consumption, the lower the rate. A fair basis for rate is an annual charge proportional to the maximum demand (not to the connected load) plus a meter rate for current actually used. An equivalent method is to charge one rate for the first 100,000 kw.-hr. in any month, a lower rate for the next 100,000, and so on.
Contract clauses should provide for interruptions of service, which may sometimes be offset by provisions releasing the purchaser from liability in case of strike or fire in his plant. The phase, frequency and voltage of current at the point of measurement should be specified. Services furnishing a high power factor should be somewhat more attractive to the vendor.
PRODUCER GAS
Operation of Producer.—The essential feature is a partial combustion of the fuel. Carbon is converted to CO, itself a combustible suitable for use in an engine or furnace. The original heat of combustion of the carbon is thereby partly converted to sensible heat. Since the gas should be cooled before use in an engine, the maximum ideal efficiency with pure air and pure carbon on the "cold gas basis" is only 0.70.
By introducing some steam as well as air, the sensible heat which would otherwise be wasted is employed to decompose steam. It then reappears as heat of combustion of hydrogen, increasing the cold gas efficiency, which is
er = [10,220(1 - a) + 6,9002/1 + (Q. + Qa + 14,650), where           x = Proportion of C burning to C02,
y = Weight of steam introduced per pound of C, in Ib.
Q8 = Heat furnished by entering steam 1 _                      ,    . ~
!:       TT   ^ f     - ,    , ,        J    .       .         > B.t.u. per pound of C. Qa = Heat furnished by entering air       J            *?    v
The value of e' may reach 0.83 to 0.85 (ideal), from which radiation losses must be deducted. Slight excesses of steam or air always lead to the formation of some C02. The "hot gas efficiency" (furnace applications) is similarly
e = [10,220(1 - x) + 6,9002/ + QJ -*• (Q* + Qa + 14,650), where Q0 = heat carried off by the gas as sensible heat, B.t.u. per pound of C.
Use of Steam.—Steam not only increases efficiency: it lowers the fuel bed temperature and thus discourages clinkering and fusion of furnace walls. It reduces sensible heat loss, but if supplied in excess decreases the rate of gasification and (ultimately) the efficiency. When the steam supply is intermittent, water gas is formed during its admission. This is so rich in hydrogen as to be objectionable for use in an engine (causing preignition) and the intermittent process is adapted mainly for furnace work or for combined furnace and engine plants. The proportions of air and steam, in the continuous process, should be kept constant for a given fuel. This is approximately accomplished by various forms of automatic regulation, but charging the fire and sudden changes in load may nevertheless produce considerable variations, reflected in the heat value of the gas. Exhaust gas from the engine may take the place of steam, the theory of operation being precisely the same. This method is less widely applied, although it leads to the formation of a gas (free from hydrogen) better adapted for efficient u*e in the engine.